1. Field of Invention
This invention relates generally to method and apparatus for producing puffed borax, and more specifically relates to a method and apparatus for producing a puffed borax product having superior particle strength and improved uniformity of hydration and puff ratio.
2. Prior Art
The preparation, properties, and uses of expanded or so-called "puffed" borax, have been described in numerous places in the literature. Excellent such discussions are set forth, e.g. in Technical Service Bulletin, No. 27, (revised September 1971) of Borax Consolidated Limited of London, England; in Bulletin 0171, entitled "Puffed Borax", issued October, 1973 by Kerr-McGee Chemical Corporation, Oklahoma City, OK; and in an article by R. C. Rhees and H. H. Hammar entitled "Puffed Borax", appearing in Soap and Chemical Specialities, Vol. XLII for January, 1966, at pages 58-61 and 118-120.
Among other things, these references describe known, prior art processes wherein particulate borax pentahydrate particles are rapidly heated to above the melting temperature of the pentahydrate, whereat the borate dissolves in its own water of hydration and the solution erupts through the partially dehydrated crystal surface, resulting in the puffed product.
The resultant puffed product thus consists of low density particles or "beads", which have high surface area and include large quantities of voids. By virtue of such structure, large quantities of liquids and/or solids can be loaded onto the expanded borax, rendering such product useful as a bulk carrier in numerous applications. For example, organics such as trichloroethylene, cyclohexanone and pentachlorophenol can be loaded at very high percentages onto the puffed borax, as can non-ionic, cationic and anionic surfactants, with the product yet retaining its free-flowing characteristics. This renders the puffed borax very useful in such diverse compositions as dairy cleaners, fabric softeners, bath additives, etc.
Various techniques have been utilized in the past to effect the desired heating of the borax feed material. U.S. Pat. No. 4,031,354 to D'Souza, for example, describes a rotary inclined tubular drier into which granular borax is fed at its higher end and puffed borax is discharged from the lower end. A countercurrent flow of dry air is maintained in the drier. Borax is prevented from adhering to the interior of the tube by an adjustable spring-loaded scraper. In this process, undesirable agglomerates tend to be formed.
British Pat. No. 629,171 describes a process of preparing anhydrous borax by calcining borax containing water of hydration in a long vertical tube furnace, the powder falling down the tube in a counter-current of hot gases which are introduced tangentially slightly above the bottom the furnace. This process utilizes gases heated to as much as 1365.degree. F., and produces a melted anhydrous borax, i.e., borax from which all water of hydration is removed, rather than puffed borax which retains water of hydration.
A considerable portion of the puffed borax presently manufactured in the U.S. has been produced by use of the so-called "borax gun," one type of which is disclosed in U.S. Pat. No. 3,454,357, to R. C. Rhees and H. N. Hammar. This device consists of a central firing chamber, typically about 31/2 feet long and several inches in diameter, through which hot gasses are passed in a given direction, with the gasses exiting through a venturi nozzle at the far end of the chamber. Compressed air is passed about the chamber periphery, which itself is mounted coaxially within a larger chamber or tube. At the venturi zone, sodium borate is deposited by gravity or other feed means. The borax particles are mixed at this venturi zone with the hot gases and carried through a further length of the enveloping tube, for a relatively short distance. Since the puffing zone (which extends from the venturi zone) in this device is of very small diameter--typically of several inches; and since the borax is introduced at the venturi zone, there is a tendency for turbulence to be present, and this introduces considerable grinding or attrition of the borax particles during the puffing process. For these and other reasons, it is found that the product produced by this type of gun includes a large number of fractured particles. The said product is accordingly found to have poor flow characteristics, and a mass of the material exhibits a comparatively high angle of repose. The presence of large numbers of fractured or partially fractured particles also leads to increased particle attrition in shipment and handling.
Perhaps because of the said turbulence, or due to other factors in the geometry of the foregoing Rhees et al apparatus, the particles of the product are further found to display a lack of uniformity in puffing ratio. By "puffing ratio" is meant the ratio between the diameter of the puffed particle and the diameter of the original feed particle which yields the puffed particle. This result is well appreciated in the prior art. See for example FIG. 1 of the aforementioned bulletin No. 171 and FIG. 11 of the Rhees and Hammar article in Soap and Chemical Specialties, wherein size distribution curves show the change in size distribution which typically results from manufacture of puffed borax by prior art methods.
Commercially available borax used as the feed for production of puffed borax is, of course, composed of particles of varying sizes. A principal reason for the change in particle size distribution in the puffed product, as is pointed out in the said references, is that by the methodologies heretofore utilized, smaller particles in the feed material tend to be preferentially puffed. This result is highly significant in that the tendency for finer feed particles to be puffed to a greater extent than larger particles, while it leads to a greater uniformity in size distribution, produces a product which for a desired average density includes high percentages of weaker highly puffed particles along with the denser less puffed particles. This type of particle population is more prone to break down by attrition when handled as during shipping. This is in contrast to a particle population wherein a relatively uniform puffing ratio is present.
The preferential puffing which occurs in Rhees et al has a further detrimental effect, in that the more highly puffed particles lose more water of hydration than the less-puffed particles. The resulting variation in state of hydration within the product renders same less suitable for use as a carrier for aqueous liquids.
It may further be pointed out that the venturi effect which is utilized in the aforementioned puffing guns, creates turbulence and undesired attrition grinding, and results in the production of fines in the resulting product. Indeed, examination of typical product yielded by use of the said gun, will reveal that same is rarely a free-flowing material--but rather contains a high percentage of dust-like material.
Yet a further difficulty with the gun-type apparatus exemplified by Rhees et al, is that turbulence generated by the venturi tends to cause uneven and unpredictable heating of the borax particles.
In U.S. Pat. No. 3,882,034, to E. J. Gibbons, a process and apparatus is described for producing puffed borax by use of a spray tower. In this instance, the borax feed material is introduced downwardly into an upwardly moving air stream. This approach while remedying some of the difficulties caused by the turbulence in gun-type apparatus, has the important disadvantage that the smaller, i.e. the lighter feed particles, which are acted upon to a greater extent by the counterflowing air stream than are the larger heavier particles, have a consequent much longer residence time in the puffing zone than do the heavier particles. This leads to precisely the undesired result which is above discussed, i.e., a preferential puffing of the smaller particles. Also to be noted, is that this tendency to preferentially over-puff small particles leads to a nonuniformity of dehydration, since in addition to over-puffing, these smaller feed particles are excessively dehydrated in comparison to the dehydration of the larger particles.
In accordance with the foregoing, it may therefore be regarded as an object of the present invention, to provide a process for manufacture of puffed borax, which produces a free-flowing, agglomerate-free, puffed borax product, which is less prone to particle attrition during shipping and handling.
A further object of the invention is to provide a process for preparing a free-flowing, agglomerate-free, puffed borax product, in which particles of borax of different sizes are puffed in a more uniform ratio, resulting in particle size distribution (PSD) curves in the puffed product which are substantially similar in shape to the PSD curves for the feed borax.
A still further object of the invention, is to provide a process for puffing a particulate borax feed stock, which avoids overheating and overpuffing of the smaller-sized particles of the borax feed, thereby assuring that such particles when puffed retain fully adequate strength and porosity.
A yet further object of the invention is to provide a process for manufacture of puffed borax from particulate borax feed, wherein the particles in the resultant product have relatively uniform water of hydration; and wherein smaller particles are not overdehydrated.
Another object of the invention is to provide a more economical apparatus and process for producing a free-flowing, agglomerate-free, puffed borax product, which has uniformly high bead strength.
A still further object of the invention is to provide apparatus for producing a free-flowing, agglomerate free puffed borax product requiring only readily available components.